This invention relates to an fibre optical device including a fibre amplifier and a reflection/transmission fibre filter coupled to said amplifier.
Erbium doped fibre amplifiers, which are seen as vital components for future optical communication systems require efficient pumping with semiconductor lasers of high optical power. However not all the pump power is absorbed in the amplifier. The remainder is usually transmitted along with the signal to the receiver. The unabsorbed pump power can be rejected by an absorbing filter or coupled out by a dichroic filter, but in either of these cases it is wasted. A further problem occurs when the amplifier is used as a pre-amplifer and the residual pump power will be of the order of 10 mW while the signal will be typically less than 1 W. Rejection of the pump to less than 50 dB of its residual power requires a very high performance filter. An additional requirement of such a filter is that it operates over a wavelength range of 20 nm so as to cover wavelength fluctuations of the pump source. Further the unabsorbed pump power will produce an excess noise in any receiver which will follow the amplifier.